1. Field of the Invention
The present invention is in the field of motion sensors, and in particular, in the field of rotation sensors and accelerometers. More particularly, the present invention is in the field of rotation sensors and accelerometers that sense electronic signals generated by a vibrating mass to detect rotation and acceleration.
2. Background Information
Locating a moving transportation device with respect to a coordinate system has been a challenge for thousands of years. The use of a compass in combination with solar or stellar readings for ships and aircraft has been replaced with inertial navigation in which the current position of a ship, aircraft or spacecraft is determined by calculations based upon acceleration and changes of direction. In order to provide the accuracy needed to correctly locate and guide a fast moving aircraft or spacecraft or to locate a submarine that is submerged and has no visual references, a highly accurate inertial navigation and guidance system is required. The need for such inertial navigation and guidance systems is well known. Typically, the inertial navigation and guidance systems include a spinning mass gyroscope or other large and heavy system for sensing the changes in direction of the transportation device on which it is mounted. Similarly, known accelerometers for providing accurate indications of the acceleration of the transportation device are also large and heavy. Furthermore, such known inertial navigation devices are susceptible to damage or becoming uncalibrated as a result of shock and vibration generated by the transportation device. Finally, such known inertial navigation devices are expensive.
In U.S. Pat. Nos. 4,524,619, 4,538,461 and 4,654,663, rotation sensors embodying vibrating masses formed from single crystals of quartz or other suitable materials are described. The rotation sensors described in those patents have the advantages of being small, lightweight, shock resistant and relatively inexpensive compared to previously known devices. Thus far, no accelerometer having comparable features has been available. Virtually all linear accelerometers in use are based on the application of a force resulting from the acceleration of a proof mass within a housing. The housing is in turn mounted to the transportation device or other object that is to be accelerated. The externally applied force that accelerates the housing of the sensor must be transmitted through some connection to the proof mass inside. In some known accelerometers, the acceleration force deflects the mass against a spring connection to the housing and the strain in the spring is detected in an open-loop manner to thereby determine the acceleration. For example, the acceleration may be determined by measuring the distance that the proof mass moves against the spring. In other known accelerometers, a closed-loop system is provided in which the deflection of the spring is counteracted by a complementary restoring force to null the deflection. For example, the complementary restoring force may be an electromagnet acting against the proof mass, or the like. The amount of force required to null the deflection is measured to provide an indication of the acceleration.
In some open-loop implementations, an incidental oscillating system in the form of a bulk or surface acoustic wave oscillator may form the spring such that a frequency shift of the oscillator is an indication of the strain in the spring.
The disadvantages of commonly used accelerometers are common to those of previously known rotation sensors in that the systems are typically large, heavy and expensive, and are subject to failure caused by shock.